In a marine seismic survey, a surveying vessel tows one or more seismic cables or streamers. Each streamer is outfitted with distributed seismic transducers, namely hydrophones, and position-control devices and position-determining sensors, such as cable-leveling birds, compasses, depth (pressure) sensors, and hydroacoustic ranging transceivers. Data from the hydrophones are sent to a controller on board the vessel via a high-speed data link, which could be an optical-fiber link. Data from the position sensors, on the other hand, are typically transmitted to the controller via a two-wire, twisted-pair line, each wire of the pair being no larger than size 22 AWG. Sensors are connected along the twisted pair in one of two ways. First, in-streamer sensors are connected in parallel directly across the twisted pair. Typically, in-streamer sensors are also powered over the twisted pair. Second, outlying sensors, such as those sensors residing in cable-leveling birds or hydroacoustic transceivers, are individually coupled to the twisted pair by means of a coupling coil connected in parallel across the twisted pair. Each outlying sensor has an individual, associated coil in the streamer.
The coupling coils in the streamer are conventionally tuned to the same frequency and typically have a fairly high selectivity, or Q, giving the two-wire communication system a narrow bandwidth and a relatively low data rate. The high Q further makes tuning of the transmitting frequency critical for effective communication. Because noise generated in the neighboring power system for the high-speed hydrophone data link occurs at a dominant frequency of about 2 kHz with harmonic level decreasing with frequency to beyond 100 kHz, typical two-wire communication takes place at about 25 kHz. Conventional communication is achieved my means of frequency-shift-keying (FSK) modulation. Other variations of angle modulation, such as quadrature-phase-shift keying (QPSK) and bipolar-phase-shift keying (BPSK), are also commonly used. Carrier frequencies on the order of 20 kHz-30 kHz are common. Proper tuning of the carrier frequency is critical to achieve a signal-to-noise ratio adequate for effective communication. Thus, present-day two-wire streamer communication relies heavily on a properly tuned system.
Prior art two-wire communication with position sensors on streamers has generally been realized by half-duplex, single-channel communication schemes. Consequently, only one sensor is allowed to send data at a time. Likewise, no sensor may send data while the controller is communicating. Such limitations have only recently become important. Several developments promise to make the half-duplex, single-channel communication system inadequate to meet expected demands in position-determining requirements. First, hydroacoustic ranging systems are seeing more widespread use. The positioning accuracy they provide, particularly in multi-streamer applications, is necessary to support the increased accuracy being demanded of seismic surveys. Each hydroacoustic transceiver typically transmits much more data to the controller than other sensors, such as compasses and depth sensors. Second, maximum streamer lengths of 10 km are expected to become commonplace, in contrast to 6 km today. Longer streamers accommodate more sensors on a single twisted pair with the concomitant increase in data traffic. Third, in the continuing quest for greater accuracy, today's typical spacings of every 300 m for depth sensors and compasses may well be replaced by spacings of 100 m, for a threefold increase in the number of these sensing devices. Fourth, to avoid interference with seismic measurement activity, availability of the communication system for data traffic may be limited to two seconds or less every seismic shot interval, which is typically ten seconds. Thus, in view of the expected expanded use of acoustics, longer streamers, closer sensor spacing, and narrower data transmission intervals, prior art two-wire streamer communication systems will be inadequate to handle the increased data traffic.
Aside from being unable to handle the increased data traffic, prior art two-wire communication systems do not predict communication failures. Failure to terminate the twisted-pair line properly causes standing waves on the line that can null out the signals at sensor positions along the line. Broken or shorted connections are another source of faulty communications. Finally, saltwater leakage causes deterioration of the communication link over time. Prior art communication systems do not recognize deterioration of the communication link until it is all but dead.
Therefore, one object of the invention is reliable, high throughput data communication over existing twisted-pair lines with many sensors distributed along marine seismic streamers up to 10 km long. It is a further object of the invention to permit early diagnosis of deteriorating communication so that prompt corrective action can be taken.